The importance of copper, as well as a variety of other metals, has led to a continuing search for more efficient and productive procurement methods. One method of copper extraction is a process of leaching, coupled together with solvent extraction, and finally copper production by electrowinning. Leaching is typically carried out by stacking the ore in piles on a prepared pad or by stacking it in a small canyon. A solution of an acid (e.g. sulfuric acid) is then applied, and as the acid solution is trickled down through the heap, copper is dissolved from the rock. The resultant copper-bearing solution (“pregnant leach solution” or “PLS”) is collected, and then transferred to the solvent extraction plant, where it is contacted by vigorous mixing with an organic solution comprising an extractant dissolved in a kerosene-like hydrocarbon diluent. In this extraction, the copper (as cupric ion) is transferred to the organic phase, where it forms a chelate-type complex with the extractant. After contact, the mixture of aqueous and organic is allowed to separate. The copper-depleted aqueous solution (“raffinate”) exits the solvent extraction plant, and the organic is transferred to stripping, where it is contacted with a strong acid solution. In stripping, a portion of the cupric ion is transferred to the aqueous phase and protons are transferred to the organic. The now copper-depleted organic is returned to extraction for re-use.
The copper-rich aqueous strip solution (“pregnant” or “rich electrolyte”) is then transferred to electrowinning (“EW”). In electrowinning, copper is plated as metal from solution at the cathode, and water is broken down at the anode to form oxygen and protons as acid.
One problem with current technology relates to iron, and possibly other undesired metals, concentration buildup in the electrowinning system. Iron is transferred from the pregnant leach solution to the EW circuit either as entrained aqueous in the organic phase or by extraction and subsequent stripping using hydroxy oximes. It should be noted that sometimes a minimum amount of iron is necessary in electrowinning solutions for various reasons. One such reason is to control the effects of manganese. When no iron is present, manganese is oxidized to permanganate at the anode as copper is plated at the cathode. When the lean electrolyte containing permanganate is returned to stripping, the permanganate, which is a strong oxidant, attacks the organic phase and damages it. However, concentrations of iron over a minimum threshold are also detrimental.
Having high concentrations of iron in electrowinning systems causes several problems, one of the most significant being loss of current efficiency. Current efficiency reflects the portion of the total supplied amps actually being used to plate the copper. In order to reduce the concentration of iron, electrolyte may be bled from the system and discarded. This discarded electrolyte solution unfortunately also contains a relatively high concentration of copper, added cobalt, and sulfuric acid, which means that these components are unintentionally lost along with the iron. The volume of this discarded electrolyte must be replaced with fresh sulfuric acid, fresh water, and cobalt. For example, bleeding electrolyte results in the reduction of cobalt concentration which is added to protect lead anodes, which can be a large expense in an electrowinning plant. Thus, in addition to the cost incurred by the drop in current efficiency, there is an additional cost in terms of lost auxiliary reagents associated with bleeding the system to control the iron concentration.
Several methods have been investigated for the removal of iron from acid sulfate systems in an effort to reduce the need for electrolyte bleeding. The most common method has been the use of specialty ion exchange resins, with the FENIX Iron Control system as the only resin currently being promoted to reduce iron concentrations in electrowinning systems. The resin utilized in this system is a sulfonated monophosphonic acid substituted crosslinked polymer. This resin material is not readily available, and is therefore associated with a very high cost. This system also has the disadvantage in that it does not offer a continuous process. That is, iron concentration cannot be continuously reduced without interruption in a given ion exchange column. As a result, several columns must be available so that some columns may be used while others are eluted and regenerated for use later on. Furthermore, the stripping conditions are relatively severe, therefore requiring highly specialized equipment, and thereby further driving up the cost.
As of yet, no solvent extraction technology has been commercially implemented to address the buildup of iron concentration. There is thus a need for methods and/or systems that address one or more of these problems.